Brewer's: Te Deum, etc.
is usually ascribed to St. Ambrose, but is probably of a much later date. It is said that St. Ambrose improvised this hymn while baptising St. Augustine. In allusion to this tradition, it is sometimes called “the Ambrosian Hymn.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894